A distance relay responds to line voltage and current inputs signals as a function of the line circuit distance between the relay location and a point of faults and determines whether a faults exist within or outside the relay operating characteristic zone between terminal A and B. The major advantage of distance relay is that the relay's operation zone is a function of the protected line circuit impedance, which is a fixed constant and relatively independent of current and voltage magnitudes. Thus the distance relay has a fixed reach. Generally if the line circuit impedance of the protected line between terminal L and R equals ZL then the relay operating zone characteristic ZI, common known as first zone, are reduces to the 70-85% of the line circuit impedance ZL with additional consideration of the remote side source impedance and fault resistance influence. As a results of that, the relay zone operation characteristic at the impedance plane is determined.
Series compensation SC is applied to worldwide EHV—Extra High Voltage transmission lines where long transmission distance and large power transmission are involved. It is an effective and economic means of increasing power transmission capability, improving power system stability, reducing transmission losses, enhancing voltage control and flexible power flow control. However, protection of such kind of line still causes problems especially for parallel lines. This is due to the fact that series capacitors shift the fault-loop impedances which results in tripping the healthy line. If the fault current and parallel circuit current flow in the same direction, the apparent impedance will be overestimated and the distance relay will underreach. On the other hand, if the fault current and the parallel circuit current flow in opposite directions, the apparent impedance will be underestimated and the distance relay will overreach. The common practice is to set each relay's reach on the basis of the predicted worst case system conditions for the particular pilot or non-pilot protection scheme in use. As a results of that considerable shortening of the first zone reach to 30-40% instead of 70-85% is applied.
An example of the distance protection relay is described in U.S. Pat. No. 5,367,426. This application based on the positive sequence impedance determination. In this solution calculated impedance converted in to magnitude angle representation are compared against load pattern and test pattern. The trip signal is send if the calculated impedance is within the test pattern.
An example of a protective relay that utilizes a parallel circuit's current in conjunctions with measurements of voltage and current on the protected line is described in U.S. Pat. No. 5,956,220. In this solution the current ratio for negative or zero sequences is used to avoid incorrect compensation of mutual coupling effect for the relays on the healthy circuit.
From patent description U.S. Pat. No. 5,796,258 are known a method and apparatus for monitoring at least one electric power transmission line for a fault condition within a predetermined distance from a measuring location. This patent describes a quadrilateral characteristic relay system for determining fault and adapts under-reach and over-reach relay characteristics.
The other protective relay system is known from patent description U.S. Pat. No. 4,896,241. The solution according to the patent describes a directional comparison relaying system including forward looking pilot relays at each terminal of a protected line segment. This is dependent on communication between different ends.
The article “Fault Location of Two-parallel Transmission Line for Non-earth Fault Using One-terminal Data”—IEEE TRANSACTIONS ON POWER DELIVERY, IEEE SERVICE CENTER, NEW YORK, US, vol. 14, no. 3, 1 Jul. 1999 (Zhang Qingchao et all) describes a fault location algorithm on parallel transmission line utilizing the local data which are three voltages and six currents. It needs line parameters data for the parallel lines. This is needed in order to eliminate the effect of remote-end in fault location calculation. The invented idea is also utilizing only local end measurements but line parameters are not needed in basic calculation. The invented idea is dedicated to other form of application.
The Article “Experimental test results for a parallel transmission lines protection scheme using wavelet transform”—IEE PROCEEDINGS: GENERATION, TRANSMISSION AND DISTRIBUTION, INSTITUTION OF ELECTRICAL ENGINEERS, GB, vol. 151, no. 6, 12 Nov. 2004, describes a scheme for the protection of parallel lines using wavelet transformer. This utilizes measurements of both current and voltage at line ends. On other hand, the invented idea is utilizing only local end information. Thus the concept of the article differs to that described in invented patent application.
Another example of the distance protection scheme is presented in the publication “Adaptive Distance Protection for Series Compensated Transmission Lines” presented at 29th Annual Western Protective Relay Conference Spokane, Wash., Oct. 22-24, 2002. In this publication in order to maintain correct directional discrimination in case of voltage reversal, a polarization function is added to the main distance protection. The polarization voltage is based on healthy phase and memorized voltage. Additionally directional determination for high-speed ground fault detection is done with utilizing superimposed currents and polarizing voltages during a short interval after the fault inception. This makes it possible to extend the reach of the first zone regardless of the capacitor presence in the fault loop, since the apparent reactance changes slowly. The presented protection scheme consists of the main distance protection, a fast tripping algorithm for multi-phase faults and a fast tripping algorithm for single phase to ground faults.
In the patent application EP11460034.9 filed by applicant, a method of selecting between internal and external faults in parallel lines using one-end measurements is presented. This method is designated for determination whether detected fault is placed on a protected parallel lines or outside the lines. This method used measurement of current signals at one end of the line in terminal A or in terminal B. In this solution current signals are processed, pre-fault and fault current signals and the zero-sequence current signal are determined, and current phasors are calculated. Then symmetrical scaling coefficient kprefault establishing a reciprocal relation between the currents of a single line of the parallel lines is determined in order to calculate corrected fault current phasors after scaling and current differences sum sdi1, sdi01, sdi2, sdi02 for each line. Then having the current differences sum sdi1, sdi01, sdi2, sdi02 for each line, the maximum sdi_max, sdi_0max and the minimum sdi_min, sdi_0min of the sum of the current differences are determined. Finally flag Internal_Fault having the conventional value of one for an internal fault within the parallel lines or zero value for an external fault outside the parallel lines is identified, or no fault conditions is identified according to the following formula:
      Internal_Fault    ⁢          (      n      )        =      {                            1                                      when            ⁢                                                                                      {                                                                                                                                                        sdi_max                          ⁢                                                      (                            n                            )                                                                                                    sdi_min                          ⁢                                                      (                            n                            )                                                                                              >                                                                                                    sdi                            thr                                                    ⁢                                                                                                          ⁢                          AND                          ⁢                                                                                                          ⁢                          FAULT_Ph                                                -                                                  Ph                          ⁡                                                      (                            n                            )                                                                                                                =                    1                                                                                                OR                                                                                                                                                                          sdi_                          ⁢                          0                          ⁢                                                      max                            ⁡                                                          (                              n                              )                                                                                                                                sdi_                          ⁢                          0                          ⁢                                                      min                            ⁡                                                          (                              n                              )                                                                                                                          >                                                                                                    sdi                            thr                                                    ⁢                                                                                                          ⁢                          AND                          ⁢                                                                                                          ⁢                          FAULT_Ph                                                -                                                  G                          ⁡                                                      (                            n                            )                                                                                                                =                    1                                                                                                            0                          otherwise                                                                    where:    sdithr—is a different current sum coefficient threshold given by the user,    n—is the real consecutive number of the present digital sample processed by the protective device,    FAULT_Ph-G(n) is a fault type coefficient indicating phase-to-ground faults,    FAULT_Ph-Ph(n) is a fault type coefficient indicating phase-to-phase fault.
In the patent application EP11460033.1 filed by applicant, a method of selecting between faulted and healthy circuits in parallel lines using one-end measurements is described. This method is designated for determination whether detected fault is placed on a protected circuit of the parallel lines or on parallel circuit. The input signals in those methods are samples of three-phase currents from both circuits of the parallel lines. In this solution current signals are processed, pre-fault and fault current signals and the zero-sequence current signal are determined, and current phasors are calculated. Then symmetrical scaling coefficient kprefault establishing a reciprocal relation between the currents of a single line of the parallel lines is determined in order to calculate corrected fault current phasors after scaling and current differences sum for each line sdi1, sdi01, sdi2, sdi02. Then having the current differences sum for each line fault, coefficients ff1, f01, f2, f02 are calculated. Next flags ff1 and ff2 having a conventional value of one or zero in order to carry out the selection of faulted and healthy circuits in the parallel lines are determined according to the following formulas:
            ff      1        ⁡          (      n      )        ⁢      {                            1                                      when            ⁢                                                                                      {                                                                                                                                                        f                          1                                                ⁡                                                  (                          n                          )                                                                    ≥                                                                                                    f                            thr                                                    ⁢                                                                                                          ⁢                          AND                          ⁢                                                                                                          ⁢                          FAULT_Ph                                                -                                                                              Ph                            1                                                    ⁡                                                      (                            n                            )                                                                                                                =                    1                                                                                                OR                                                                                                                                                                          f                          01                                                ⁡                                                  (                          n                          )                                                                    ≥                                                                                                    f                            thr                                                    ⁢                                                                                                          ⁢                          AND                          ⁢                                                                                                          ⁢                          FAULT_Ph                                                -                                                                              G                            1                                                    ⁡                                                      (                            n                            )                                                                                                                =                    1                                                                                                            0                          otherwise                                                                    for line circuit #1 which is the protected line,                and        
            ff      2        ⁡          (      n      )        ⁢      {                            1                                      when            ⁢                                                                                      {                                                                                                                                                        f                          2                                                ⁡                                                  (                          n                          )                                                                    ≥                                                                                                    f                            thr                                                    ⁢                                                                                                          ⁢                          AND                          ⁢                                                                                                          ⁢                          FAULT_Ph                                                -                                                                              Ph                            2                                                    ⁡                                                      (                            n                            )                                                                                                                =                    1                                                                                                OR                                                                                                                                                                          f                          02                                                ⁡                                                  (                          n                          )                                                                    ≥                                                                                                    f                            thr                                                    ⁢                                                                                                          ⁢                          AND                          ⁢                                                                                                          ⁢                          FAULT_Ph                                                -                                                                              G                            2                                                    ⁡                                                      (                            n                            )                                                                                                                =                    1                                                                                                            0                          otherwise                                                                    for line circuit #2    where:    fthr—is threshold for fault coefficients ff1, fi01, f2, f02 given by user,    FAULT_Ph-G1(n) is a fault type coefficient indicating phase-to-ground faults and in the line #1,    FAULT_Ph-Ph1(n) is a fault type coefficient indicating phase-to-phase fault in the line #1 or non-fault conditions,    FAULT_Ph-G2(n) is a fault type coefficient indicating phase-to-ground faults and in the line #2,    FAULT_Ph-Ph2(n) is a fault type coefficient indicating a phase-to-phase fault in the line #2 or non-fault conditions.
Next the selection of faulted and healthy circuits in parallel lines is performed in the following actions.
First it is checked if only one of the coefficients ff1 or ff2 equals one, and if:                ff1(n)=1 AND ff2(n)=1 then ff1(n)=0 AND ff2(n)=0,        or        ff1(n)=0 AND ff2(n)=0,it means that the outside fault of the parallel lines has been identified.        
Next the relation between the value of the coefficients ff1 and ff2 is checked, and if                ff1(n)=1 AND ff2(n)=0,it means that internal fault within line circuit #1 of the parallel lines has been identified;or if        ff1(n)=0 AND ff2(n)=1,it means that internal fault within line circuit #2 of the parallel lines has been identified.        